Semi-Conductors

Bipolar Junction Transistors

Fundamentals of Operation

The reverse saturation current can be controlled by injecting minority carriers into the junction

A forward biased junction is used to inject the minority carriers

To ensure that almost all injected minority carriers are swept across the reverse biased junction, the base should be short compared to diffusion length and carrier lifetime should be long\(W_b << L_p or L_n\)

The Emitter injects minority carriers into the Base. These carriers are swept across the reverse biased junction to the Collector.

To ensure more carriers are injected into the base than the emitter the emitter must be doped heavily compared to the base

Base Current \(I_B\)

Current flowing out of the base and not into the collector

Good transistors will limit this current

There must be some recombination of injected holes in the base, electrons lost to recombination must be supplied through base current

Some electrons will be injected into emitter from base, these are supplied by \(I_B\)

Some electrons are swept into base from B-C due to thermal generation in collector. This is small and reduces \(I_B\)

Amplification with BJT

Currents at emitter and collector can be controlled by the small base current

Collector Current \(i_C\)

Basic collector current is completely made of the carriers injected from the emitter that are not lost to recombination\(i_C = B i_E\)

\(B\) Base Transport Factor: Fraction of injected carriers that make it to the collector